1. Technical Field
The invention relates to an imaging lens mounted on an imaging device, such as a digital still camera, a cellular phone with camera, PDA (Personal Digital Assistance), including an imaging element such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor).
2. Description of the Related Art
In the field of an imaging element such as CCD and CMOS, a reduction in size and an increase in numbers of pixel are proceeding greatly nowadays. Therefore, a small and high performance type is required for an imaging device itself and lenses mounted on the imaging device. For the purpose of a reduction in size, reduction in a total length and reduction in height (size reduction in the diameter direction perpendicular to the optical axis) are required. Also, normally the telecentric property, i.e., an incident angle of the principal ray to the imaging element should be set closely in parallel to the optical axis (incident angle on the imaging plane comes close to zero with respect to a normal line to the imaging plane), as well as the reduction in size is demanded in the imaging optical system. In order to ensure the telecentric property, it is preferable that an optical aperture diaphragm is arranged as closely as possible to the object side. JP 2005-292235 A and JP 2004-240063 A (corresponding to US 2004/150893 A) disclose an imaging lens having such a configuration that three lenses are provided as a whole and that an optical aperture diaphragm is arranged on the most object side.
By the way, as number of pixels in an imaging element increases, a mechanical shutter is provided in an imaging device for capturing a still image in order to attempt to reduce a signal noise in the imaging element. When the shutter is provided, it is preferable that the mechanical shutter is arranged near an optical aperture diaphragm to reduce unevenness in a quantity of light. In contrast, in the imaging lens having a three-lens configuration, as described above, it is preferable that an optical aperture diaphragm is arranged as closely as possible to the object side, e.g., in front of or in rear of a first lens, in order to ensure the telecentric property. However, it is disadvantageous in size reduction that the shutter mechanism is arranged on the most-object side, e.g., in front of the first lens. Therefore, it may be considered that the shutter mechanism is arranged inside the lens system, e.g., between the first lens and a second lens. For that purpose, it is demanded to develop a lens having high aberration property compatible with increase of number of pixels while ensuring a sufficient air interval between the first lens and the second lens in the three-lens imaging lens in order to arrange the shutter mechanism. Example 3 in JP 2005-292235 A provides such a lens configuration that an interval between the first lens and the second lens is ensured relatively widely, which is advantageous in arrangement of the shutter mechanism. However, it is further demanded to develop a lens that is more advantageous in arrangement of the shutter mechanism and has high aberration property.